Thanks for the detailed information, it's helpful for troubleshooting. I included some information and questions below to help resolve your issue:
After reviewing your order history, I see you bought your refill carts in 2010, and have bought a total of three bottles of VM ink (one every two years, the most recent in Sept 2014). 8oz of ink is the equivalent of using one cartridge worth of ink in two years, which is infrequent use of a pro model printer.
I have been using the exact same lot# VM Ink in our studio production printers (one 9880 + two 9900s), for nearly a year without any major issues (though I refill carts every few months, shake carts at least every other week, and use/clean printers regularly). It's common for me to see a few missing and/or mis-firing nozzles after the printers sit unused for a few days, but everything looks perfect again after a regular cleaning cycle, so I don't suspect your problem is specific to this ink batch, or that the ink is bad. I do know that magenta (and yellow) pigment ink is often the thickest of the colors, therefore can settle faster and get particle build up in the damper faster that other colors (this is true with all pigment inks, including Epson).
I checked my bottles + carts, and don't have excessive/abnormal build up on the sides like you report, which I agree sounds strange/alarming. Can you send a photo of your magenta cartridge, showing how ink is sticking to the sides for me to see? Are you using the cartridges with the air vent hole open or with the filter?
Have you ever replaced the dampers in this 4880 printer? If so, about how long ago? If the dampers have never been replaced, then that's something you should do (every 1-3 years, along with your wiper blade and cap pump assembly). Helpful cleaning + maintenance info can be found here: http://www.inkjetmall.com/tech/content.php?133-Printer-Cleaning-and-Preventative-Maintenance
Here's my suggestions: To clean out the internal ink line, remove the magenta cart and install a cart containing PiezoFlush. You'll need a syringe (30ml or larger) to manually draw flush/ink thru the line, as well as the repair manual for instructions on how to dismantle + replace parts in your printer (if you don't already have it, you can get the repair manual from www.2manuals.com), and at least one new damper (but if you've never replaced dampers in this printer, you should replace all 8 at once). From what you explain, I believe your issue is in the magenta damper, but depending on how long this has been going on, you may have also developed some drying/clogging of the print head channel- in which case, you would benefit from having our print head cleaning kit on hand, to gently flush the magenta channel when you have access to the head. Also- have paper towels handy, because you will have some amount of ink drips.
* Raise/unlock the cartridge locking levers and slide all carts out about 1" (to stop ink flow). Disconnect the dampers from the print head. I suspect the magenta damper screen is clogged, so don't expect you'll be able to draw thru the old damper. Disconnect the magenta damper from the ink line, then lower the left bay cartridge locking lever (to open ink flow), and use the syringe to draw ink out of the magenta line, until you get air. Unlock the cartridge lever and install the magenta flush cart, then lower the lever again. Now, use the syringe to draw PiezoFlush thru the line, and if you feel it needs to soak, raise the lever again to let the line soak with flush fluid.
Once you feel the line is flushed, install the magenta ink cartridge (emptied + filled with fresh ink, just to be safe), and repeat the procedure to refill the line with ink. Attach the new damper(s), and prime with ink using the syringe. Always raise/unlock cartridge levers to stop ink flow when parts are disconnected (except the short time you need to draw with the syringe), to avoid an ink mess.
* To flush the print head channel using our print head cleaning kit, follow the instructions (which are for desktop model printers) by placing a folded paper towel under the print head, then gently injecting PiezoFlush thru the magenta channel (no need to flush other channels if they're all working well). If there's resistance- pull the syringe plunger up to help dislodge the blockage. If any crud comes up into the syringe, remove + empty it, then refill with fresh flush fluid and repeat until you have good flow thru the channel. Clean the bottom of the head when removing the paper towel.
*After cleaning the head channel, flushing the ink line and attaching the new damper(s), reassemble your printer, then do 2-3 regular cleaning cycles and print a nozzle check to see how things look.
Some helpful links:
1. Print head flush kit: http://shopping.netsuite.com/s.nl/c.362672/it.A/id.6605/.f?sc=18&category=68411
2. Syringe: http://shopping.netsuite.com/s.nl/c.362672/it.A/id.7297/.f?sc=18&category=68411
I hope this helps, keep me posted.